1. Field of the Invention
The present invention relates generally to electrical systems and more particularly to short circuit and ground fault protection for electrical systems.
2. Description of the Related Art
When electrical power in an electrical system is provided from an outlet, a potential hazard of electric shock exists for a person who plugs a device into the outlet or unplugs a device from the outlet. One such hazard involved is that the person accidently touches a "hot" conductor of the electrical system. A "hot" conductor is one which is at a non-zero voltage. If the person touches a hot conductor, the person can present an inadvertent path to ground (a "ground fault") for electrical current in the hot conductor. If a single-phase electrical system is at issue, there is a single hot conductor. In a three-phase electrical system, on the other hand, there are three hot conductors.
A device which can be employed to detect a ground fault is a ground fault circuit interrupter (GFCI). A GFCI measures a sum of the currents in all current-carrying conductors in the electrical system. If the currents do not add to zero, the GFCI concludes that some current is being shunted through an inadvertent path to ground. The GFCI then quickly opens the current-carrying conductors. GFCIs for single-phase systems are commonly used in household electrical systems. Further, U.S. Pat. No. 4,685,022, issued to Nichols, III et al., discloses a GFCI for a three-phase electrical system.
However, there is a hazard which a GFCI cannot protect against. A person can accidently place himself between two current-carrying conductors. In a single-phase system, that would be manifested by the person placing himself between the hot conductor and the "neutral" (or return) conductor. In a three-phase system, the situation would be manifested by the person placing himself between two hot conductors. Where such an inadvertent current path exists between two current-carrying conductors (referred to herein as a "short circuit"), the sum of the currents in the current-carrying conductors of the electrical system is still zero. As a result, a GFCI would not be able to detect that an inadvertent current path exists. However, an electrical shock hazard clearly exists in this case.
A fault detection system which detects inadvertent current flow both between a hot conductor and ground and between two current-carrying conductors would therefore provide an advantage over the prior art. The advantage would be a reduced hazard of electric shock from inadvertent current flow between current-carrying conductors.